I think that @Jimmy V. said it best- reread what he posted at least a couple of times. Remember that the 4 cam Speedsters had 110-120 hp (out of 1500 cc's at about 6500 rpm so there really wasn't much below 4,000 or 4500) so a 1915 with 110 hp (at a slightly lower redline and with more bottom end/lower midrange) with a 4.12 transaxle will be a pretty peppy car around town and with stock 1st-4th gears will still allow you to cruise on the highway at 70 mph. That said- there's nothing like the power of a 2110, 2180 or 2332 in these cars- any of these engines just makes it that much more fun!
You could use a 3.88 r&p with a 1915 for a slightly lower rpm on the highway, but you will be trading off that around town "peppiness". Of course, a larger engine with the 3.88 will restore most of that off the line acceleration while still giving the lower cruising rpm's- with a 4 speed there are compromises, and you can't have it all. A 5 speed, on the otherhand, just ups the game a whole 'nother level...
Hope this helps. Al